Heatsink Thermal Resistance Calculator

Sizing a heatsink from a thermal resistance budget

Heat flow from junction to ambient follows the same math as a resistor divider carrying current: each interface in the path — junction-to-case, case-to-sink (through the thermal interface material, e.g. pad or grease), sink-to-ambient — has a thermal resistance in °C/W, and they add in series. The total temperature rise above ambient is ΔT = P × (R_th(j-c) + R_th(c-s) + R_th(s-a)), exactly the thermal equivalent of Ohm's law (P ↔ I, °C ↔ V, °C/W ↔ Ω).

Given a target maximum junction temperature, everything except R_th(s-a) — the heatsink's own resistance to ambient — is fixed by the device and the interface material. Solving for it: R_th(s-a) = (T_j(max) − T_amb) / P − R_th(j-c) − R_th(c-s). Any heatsink with a datasheet R_th at or below this value (in its intended airflow condition — natural convection numbers don't apply if the sink is forced-air cooled, and vice versa) keeps the junction under its rated maximum at this power.

Don't design to the limit: leave headroom below T_j(max) for ambient swings, aging thermal paste, and part-to-part variation in R_th(j-c) — the margin field above derates the target rise before solving for R_th(s-a), so a larger margin gives you a more conservative (lower) required heatsink resistance. If the required R_th(s-a) comes out negative, the device can't dissipate P at that ambient with the given interface — you need a lower-R_th(j-c) package, better case-to-sink coupling, forced air, or a lower power budget before a heatsink can help.